Polyethersulfones, polyamideimides, polyetherimides and polyimides not only have excellent heat resistance, but also exhibit good adhesion to metals and other polymers such as fluororesins, and are therefore used as film-forming agents.
One example of a method for this type of use is a known method that is employed when the surface of a metal substrate is to be coated with a fluororesin, wherein prior to the application of the fluororesin, a resin solution composition prepared by dissolving a resin such as a polyethersulfone in a solvent is adhered to the metal substrate, thereby forming a primer layer of the resin solution composition on the substrate surface.
Examples of known solvents for the above resin solution composition include solvents having Hansen solubility parameters that include a dispersion parameter δD (units: MPa0.5) of at least 14.7 but not more than 20.3, a polar parameter δP (units: MPa0.5) of at least 4.5 but not more than 18.4, and a hydrogen bonding parameter δH (units: MPa0.5) of at least 4.3 but not more than 15.7. Disclosed examples of such solvents include sulfolane, N,N-dimethylformamide and γ-butyrolactone. However, resin solution compositions that use any of these solvents individually suffer from problems such as gelling and turbidity caused by the formation of precipitates (see Patent Documents 1 to 3).